Childhood intelligence On average, 20 to 40 per cent of a child's intelligence is inherited, an international team of researchers has found.

The finding, from the largest yet genetic study of childhood intelligence, was published recently online in the journal Molecular Psychiatry.

"In our study, we found that between 20 to 40 per cent of the variation in childhood IQ is due to genetic factors," says lead author and geneticist, Dr Beben Benyamin, from the University of Queensland.

"This estimate from DNA information is lower than family studies, but it is consistent with the conclusion childhood intelligence is heritable."

Benyamin says previous studies of twins and families have suggested genes might account for 40 per cent of childhood intelligence.

But, he says, such conclusions are controversial because it is not possible to be separate out the effect of environment.

"There's a lot of criticism about the validity of the conclusions," says Benyamin.

To counter this limitation of family studies, Benyamin and colleagues used genetic data from Australia, the UK, the Netherland and the US.

They analysed DNA samples from nearly 18,000 children, aged six to 18 years, along with their IQ scores.

The researchers wanted to see if they could correlate any patterns of differences in the DNA with patterns of differences in IQ.

They found that a gene, called FNBP1L, previously reported to be the most significantly associated gene for adult intelligence, was also significantly associated with childhood intelligence.

But when looking for genetic factors influencing intelligence, and other traits, scientists prefer to look for single-nucleotide polymorphism (SNPs), or gene variants as these provide more precise genetic information.

As with similar studies in adults, the study did not find any single gene variant SNP that was strongly associated with childhood intelligence, says Benyamin.

"But when we looked at the combined effect of all SNPs we can estimate the contribution of genetics to be about 20 to 40 per cent of the difference in IQ," says Benyamin.

He says this estimate is lower than that suggested by studies of families and twins and could be due to the fact that only common SNPs were analysed. There might be other genetic variations not accounted for in the study.

Benyamin says the study suggests many genes contribute to childhood intelligence and that each provides a small effect.

Larger studies involving hundreds and thousands of individuals could help narrow down the contribution of genes to childhood intelligence, and may also identify which individual SNPs are most strongly associated.

Benyamin says understanding the factors influencing intelligence is important since IQ is a good predictor for education, income and lifespan.

He says the findings may also help in better understanding intellectual disability.

Other studies show that genetics contributes 40 to 50 per cent to adult intelligence and the reason for this higher genetic effect is debatable, says Benyamin.

According to senior author of the study, Professor Peter Visscher, also of the University of Queensland, twin and family studies suggest a possible reason for this higher contribution.

"The usual explanation is that non-genetic factors, such as shared environmental factors between family members (e.g. which school you go to) are more important at a young age, and that the effects of genetics become more important later in life," says Visscher.